Old National Semiconductors JFET attenuator

[knutolai]

Browsing this old National Semiconductors http://www.ti.com/lit/an/snoa620/snoa620.pdf appnote I stumbled over the JFET attenuator below.


Is anyone familiar with any effect circuits implementing this design? Would it be correct to regard the JFET in such a application as purely a voltage controlled resistor? The appnote says it's a VCR, but I've tried hooking a J201 up to a variable voltage source and measured the resistance across it with no sensible results. What am I missing?

[midwayfair]

It's used in the Clinton bypass:



(Image from Geofex)

I'm pretty sure you can get a lot more than 800K out of a JFET!

Obviously it has some applications with control voltages (envelope, LFO, etc.), assuming you can do it without distortion. I've tried something like this before with an LFO and there was a lot of distortion across the FET. I think ~arp might have tried it with something at some point this year as well, but I don't remember what that would have been.

[R.G.]

Is anyone familiar with any effect circuits implementing this design? Would it be correct to regard the JFET in such a application as purely a voltage controlled resistor? The appnote says it's a VCR, but I've tried hooking a J201 up to a variable voltage source and measured the resistance across it with no sensible results. What am I missing?

What you're missing is that it is only a linear voltage controlled resistor for very small voltages across it. For a normal JFET, this amounts to perhaps 1/2 volt peak audio. For the J201, which is decidedly not a normal JFET, it is much smaller. So it was probably working as expected, but only for tiny voltages.

I'm pretty sure you can get a lot more than 800K out of a JFET!

The app note was saying 800 ohms, not 800K. They were describing the minimum resistance of the channel, not the maximum.
But yes, all JFETs will turn completely off, equivalent to many megohms.

[amptramp]

If you look at the circuit, the JFET drain is connected to a virtual ground.  The op amp is powered by positive and negative supplies and in the linear range, it drives the inverting input to nearly the same voltage as the non-inverting input which in this case is zero.  If the ground was replaced by 1/2 Vcc and the op amp was powered by Vcc and ground, you would get the same result.  Thus, when it is on at high gain, the JFET drain sees a constant voltage and the source is connected to the incoming signal.  At high gain, the resistance of the FET is low, so signal distortion would be minimal.  At lower gains, as R.G. has stated, the signal is added to the gate voltage and the gain varies with the sum of the signal and input voltages, which is Vgs.